Engine exhaust systems utilize emissions control devices to treat exhaust gas of internal combustion engines, reducing the amount of particulate emissions released to atmosphere. Emission control devices typically include particulate filters and catalytic converters. For example, three way catalytic converters (TWC) that are capable of reducing NOx may be included in the emission control device. In addition, particulate filters (PF) may be often positioned downstream of the TWC to collect particulate matter, including carbon particles from incomplete combustion (e.g., soot and ash). As particulate matter accumulates in the filters, there is a gradual increase in restriction of exhaust gas flow. When the particulate filter becomes sufficiently loaded with soot, an unfavorable exhaust backpressure may be generated that can adversely affect combustion, emissions, and engine performance.
To reduce exhaust backpressure related issues, particulate filters may be periodically regenerated. For example, the filter may be regenerated opportunistically during vehicle driving. For example, during conditions when the engine is operating in a deceleration fuel shut-off (DFSO) mode, the exhaust gas may passively reach regeneration temperatures (e.g. above 450° C., for example). Flow of this hot exhaust gas through the particulate filter can cause at least some of the carbon particles accumulated in the filter to incinerate, and the combustion products may be expelled to atmosphere. However, passive regeneration opportunities may be infrequent since they rely on DFSO occurrence. In one example, if DFSO events do not occur frequently due to the vehicle operating at high loads for an extended duration, filter regeneration may be compromised. In additional examples, the particulate filter may be actively regenerated, wherein engine operation may be intrusively adjusted to raise exhaust temperatures and facilitate soot incineration. For example, the engine may be operated rich and lean (relative to stoichiometry) sequentially to actively burn off the soot. In still further examples, fuel may be injected into the exhaust proximate the filter while the engine is operated lean so as to burn the soot locally at the filter. However, the reliance on rich/lean engine operation during active regeneration can result in a drop in fuel economy.
Still another attempt to manage the soot load of a particulate filter includes routing exhaust gas through the exhaust treatment system to an engine intake via an exhaust gas recirculation (EGR system), as shown by Lupescu et al. in U.S. Pat. No. 8,635,852. Therein, Lupescu discloses an exhaust treatment system configured to purge stored hydrocarbons and particulate matter from a trap assembly including a particulate filter to an engine intake via an EGR passage of the EGR system.
However, the inventors herein have recognized potential issues with such systems. As one example, it may be difficult to reconcile the purge flow required to regenerate the filter with EGR flow requirements. In particular, based on engine operating conditions, an EGR valve may be frequently and rapidly transitioned between a high flow setting (e.g., a setting that provides 7-20% mass flow) and a low flow setting (e.g., a setting that provides 2% mass flow). The rapid and significant change in EGR flow setting allows pumping losses to be reduced, improving the fuel economy associated with EGR usage. However, the same change to the flow setting for purged HCs can result in combustion instability. As another example, in the approach of Lupescu, filter regeneration may not be completed if there are intermittent torque transients. For example, if there is a tip-out to idle conditions while the filter is being purged to the intake, the EGR valve may be closed since the EGR tolerance of the engine at idle conditions is low, and further due to the interference of the EGR valve opening on manifold vacuum generation. As a result, further purging may of the filter may terminate, even though the purge tolerance of the engine at the idle conditions is higher than the EGR tolerance.
In one example, the issues described above may be addressed by a method comprising: while spinning an engine fueled, purging soot from a loaded particulate filter to an engine intake using reverse flow of exhaust gas through the filter; and while spinning the engine unfueled, regenerating soot from the loaded particulate filter and flowing to a tailpipe using forward flow of air through the filter. In this way, purging and regenerating may be performed in coordination with one another such that soot loading of the particulate filter may be reduced as efficiently as possible.
As one example, an exhaust system may include an emission control device having a three way catalyst (TWC) upstream of a gasoline particulate filter (GPF). The exhaust system may include an exhaust manifold leading to a branched exhaust passage system including multiple exhaust valves for controlling a direction of exhaust flow through the emission control device. For example, a first exhaust passage may direct exhaust from the exhaust manifold to the emission control device via an exhaust turbine and/or via a waste-gate passage bypassing the turbine. The exhaust may flow to a tailpipe upon flowing in a forward direction through the emission control device (that is, from the inlet to the outlet of the GPF and thereon through the TWC). A further bypass exhaust passage may be coupled to the first exhaust passage, downstream of the turbine, for directing exhaust gas to the tailpipe while bypassing the emission control device. A bypass valve coupled at a junction of the first exhaust passage and the bypass passage may be used to direct exhaust through the first exhaust passage or the bypass passage. A second exhaust passage may couple the exhaust manifold to the first exhaust passage at a location downstream of the TWC. A flow selector valve coupled at a junction of second exhaust passage and the first exhaust passage may be used to direct exhaust to the tailpipe through the first exhaust passage (while bypassing the emission control device) or into the outlet of the GPF so as to enable reverse flow of exhaust through the GPF. A third exhaust passage may couple the inlet of the GPF to an intake manifold, at a location downstream of an intake compressor. The third exhaust passage may include a recirculation valve which is used to direct exhaust drawn from the inlet of the GPF into the intake manifold. At any given time, a larger portion of the total exhaust flow from the exhaust manifold may be directed into the first exhaust passage relative to the second exhaust passage.
Based on engine operating conditions, a position of each of the valves may be adjusted to enable one of forward flow, reverse flow, or bypass flow through the emission control device. For example, during conditions when neither soot loading or unloading is required at the GPF, the valve positions may be adjusted to enable exhaust to be directed into the bypass passage, while bypassing the GPF. As another example, during conditions when soot loading is required at the GPF, the valve positions may be adjusted to enable all the exhaust to be directed into the first exhaust passage and from there to flow in a first, forward direction through the GPF (from an inlet to an outlet of the GPF and from thereon into the TWC) wherein exhaust PMs are stored on the GPF. At this time, no exhaust is directed into the second exhaust passage. Also, during conditions when soot unloading is required at the GPF and a DFSO opportunity is available, the valve positions may be adjusted to enable the first, forward directional flow through the GPF via the first exhaust passage so that passive regeneration can be advantageously used to incinerate soot at the filter. During conditions when soot unloading is required at the GPF and a DFSO opportunity is not available, such as when the engine is running fueled, the valve positions may be adjusted to enable a portion of the total exhaust flow (e.g., 10-20% of the total flow) to be directed into the second exhaust passage and from there to flow in a second direction, opposite the first direction, through the GPF (from the outlet to the inlet of the GPF) before flowing the exhaust to the intake manifold. Herein exhaust PMs are drawn off the lattice structure of the GPF during the exhaust flow and are directed into the engine intake manifold where they are incinerated along with injected fuel during cylinder combustion.
In this way, particulate filter cleaning may be improved. By flowing at least a portion of exhaust through the filter in a reverse direction (in a filter loading direction), soot collected on the filter may be gradually removed from the filter and purged to an engine intake manifold. By adjusting exhaust system valves to enable the reverse flow to occur while the engine is operating fueled, the filter may be cleaned over a larger portion of a drive cycle, albeit at a slower rate. By providing the reverse flow via a dedicated GPF recirculation passage and GPF recirculation valve, the higher tolerance of the engine for low purge flow can be leveraged to purge the filter slowly, but more completely, without affecting combustion stability or manifold vacuum generation, particularly at idling conditions. By also enabling forward flow through the filter during conditions when the engine is operated unfueled, such as a DFSO, the filter can also be cleaned opportunistically at a higher rate. By coordinating the removal of soot using reverse flow through the filter with the removal of soot using forward flow through the filter over different conditions of a drive cycle, a more complete cleaning of the filter can be achieved, reducing the need for active regeneration of the filter, and the associated fuel penalty. By also adjusting the recirculation of exhaust gases to the engine intake based on the ingestion of soot from the GPF at the engine, disruption of filter regeneration due to sudden torque transients can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.